Many industries utilize systems that hold or process a pressurized fluid. Each such system typically includes a safety device designed to prevent the over-pressurization or under-pressurization of the system. In an emergency situation where the pressure differential between the system and the atmosphere endangers the physical integrity of the system, the safety device will create an opening to provide a vent path to relieve the pressure differential within the system. The opening allows fluid to flow into or out of the system to reduce the magnitude of the pressure differential in the system.
Some systems, such as, for example, systems used in the food processing industry, require protection from both over-pressure situations and under-pressure situations. In these types of systems, an under-pressure situation, or vacuum, within the system can damage sensitive equipment. In the food & drug processing industries, for example, a vacuum may be created in a system when the system is being cleaned after a completion of a process. Typically, these types of systems are cleaned and/or sterilized with a steam spray apparatus that removes any product or contamination from the system after a processing operation is completed and before the next processing step or cycle begins. If the system is not properly controlled during steam cleaning, a sudden vacuum can be developed, which may cause damage to the system. For example, if cold water were introduced to the system while steam cleaning, the steam may condense and create a vacuum situation.
Thus, to completely protect such a system, the pressure release device must provide two-directional pressure relief. The first direction of pressure relief prevents damage or safety hazards resulting from an over-pressurization, or a positive pressure differential situation. The second direction of pressure relief prevents damage or safety hazards resulting from under-pressurization, or a negative pressure differential situation. Since pressurized systems and atmospheric storage systems are typically designed to withstand a greater positive pressure differential than a negative pressure differential, an appropriate two-directional pressure relief device should have the ability to function when exposed to significantly different pressure differentials.
It should be noted that some systems are unlikely to encounter an over-pressure situation and, thus, the only risk is exposure to a negative pressure differential. In these types of systems, a pressure relief device need only protect the system from a negative pressure differential.
Some systems require pressure protection at very low levels, measured in “inches of water column” rather than “pounds per square inch.” This type of low pressure protection may be required in both over-pressure and under-pressure directions, or just in one direction.
A typical two direction pressure relief device includes a sealing member that is sealingly engaged with the system. The sealing member is surrounded by a pair of support members. One support member releases the seal when the seal is exposed to a predetermined positive pressure differential and the other sealing member releases the seal when the seal is exposed to a predetermined negative pressure differential. To provide protection from a negative pressure differential only, the positive pressure support member may be omitted.
The positive pressure support member provides a backdrop for the sealing member and is configured to withstand a predetermined force. As the positive pressure in the system rises, the seal moves against the positive pressure support. When the pressure reaches a predetermined level, the positive pressure support releases the seal to create a vent path and reduce the pressure in the system. Typically, the positive pressure support member is a generally solid unit that has a series of holes, slits, or perforations. The holes allow fluid to enter the system if the seal releases under a negative pressure differential and the slits allow the support member to open when the positive pressure differential reaches a predetermined level. However, when opening in the negative pressure differential direction or in low pressure single direction applications, the positive pressure support does not always fully open, which results in an obstructed flow path for the venting fluid.
The negative pressure support, often referred to as a “girdle,” is typically disposed between the system and the seal. When a light negative pressure differential acts on the seal, the seal moves towards the system and into contact with the girdle. The girdle buckles when the seal experiences a negative pressure differential. The amount of girdle buckling is directly related to the magnitude of the experienced pressure differential. The same girdle and seal combination may be used to provide a single direction low pressure relief device for either positive or vacuum relief.
However, the force of the negative pressure differential on the seal and girdle arrangement may not physically open the seal. Thus, a knife blade may be positioned adjacent the girdle to puncture the seal when the girdle buckles sufficiently under the negative pressure differential. If the girdle buckles progressively, as opposed to instantaneously, the knife blade may gently tear the seal providing a very small pressure relief path.
The flow path created through the seal may depend upon the size of the opening in the seal and the configuration of the positive pressure support. The larger the opening in the seal, the greater the flow path through the pressure relief device. However, the positive pressure support does not open when the seal opens under a negative pressure differential and thus acts as an impediment to fluid flow. The positive pressure support may include openings, or perforations, that allow fluid to flow through the seal under these conditions. The positive pressure support may limit the size of the created flow path to about 50% of the nominal cross sectional area of the pressure relief device.
In light of the foregoing, there is a need for a pressure relief device that (1) provides a high flow area for both positive and negative pressure releases; (2) consistently opens at a predetermined pressure differential in both the positive and negative directions; (3) provides a two-way device that operates at low pressures in both directions or at widely different set pressures in each direction; and provides reliable and improved opening in a low pressure direction.